This invention relates to the field of membrane switches and, more particularly, to the provision of a spacer element for a membrane switch which will allow air trapped beneath the top film to move to and from the switch areas during switch actuation or during changes in atmospheric conditions.
In prior art membrane switches, some structures have included upper contacts on a flexible membrane which are selectively pushed down by the operator's finger to contact corresponding contacts on a lower film or printed circuit board. Other structures have included an upper conductive member or film which is forced down to bridge or short together two or more contacts on the bottom member. Various means have been used to prevent unwanted contact, the most common being an insulating film such as the commercial product known as Mylar, with an aperture for each switch area. Since the spacer film is adhesively attached to the upper and lower layers having contacts thereon, the switch actuating force must work against the pressure of the air or other gas trapped between the contacts. In some cases, air was forced out of a contact area and beyond the adhesive and, since the air could not return, the contact layers tended to remain in permanent contact. This condition could occur after normal usage or after testing at elevated temperatures. Attempts have been made to let air flow from a depressed contact area to an undepressed area by providing channels in the spacer, or alternatively, in the upper or lower contact layers. These attempts either do not allow enough air to move quickly enough, or require special shaping of the top or bottom layer.
In a passenger-operated automobile seat cushion switch, a relatively thick apertures foam layer spaces two groups of common contacts until they are forced into contact by an occupant of the seat. Since the spacer of this switch is compressed over most of its area simultaneously in order for the contacts to touch, it must be made of the ordinary open-cell highly resilient foam. Since the contacts of this switch do not require a high degree of environmental protection, this material causes no problems.
In a membrane switch, very little, if any, resilience is required in the spacer, since only a small portion of the upper film layer is depressed to close a contact. Therefore, polyurethane "felts" were tested as spacers. A felt is made from a sheet of foam which is compressed between heated platens in order to achieve greater density and firmness. Other characteristics are changed little, if at all. Membrane switches have been designed using spacers made of open-cell reticulated foams, but when the switches were environmentally tested, it was determined that airborne contaminants could too easily reach the switch areas and cause corrosion of the contact surfaces. Closed-cell foam, on the other hand, provides some resilience when a contact area is depressed, but does not allow movement of air to and from the area and can also cause severe expansion problems under elevated temperature conditions. Attempts have been made to solve these compression and expansion problems by cutting channels into the foam layers connecting switch contact areas. While such channels solve a portion of the problem, foam layers cut in this way have proven to be difficult to handle during switch assembly.